Please use the -splitscreen parameter from the command line. If you go into /docs and read the text files, it will explain how to use it. It is playable but the input setup needs work. All of the features mentioned here actually work, and if you need more validation you are more than welcome to visit the official 3DGE forums, which is also contained in the README.
A solid DOOM source-port with decent features, although completely overshadowed by much more powerful and impressive ports out there. One of the features that caught my eye for this one was the splitscreen feature, which of course does not seem to actually exist within the game itself. Also no documentation as to how it's supposed to work. So the feature listings could potentially be false as well. With no real unique features of it's own, it really doesn't have much to offer to Doom players that other ports don't already do better.